1. Field of the Invention
The invention relates to routing signals through communication networks.
2. Description of the Related Art
A star network is a network of nodes having a topology in which one of the nodes (called “the central node”) is directly connected to every other node in the network, while each of those other nodes is directly connected only to the central node.
One advantage of star networks is that traffic can be transmitted between any two nodes in the network with at most two “hops,” where a hop is defined as transmission between two directly connected nodes. For every hop in a connection, the node at the beginning of the hop performs a certain amount of signal processing to route the signal towards the node at the end of the hop. The greater the number of hops in a connection, the greater the total amount of such processing overhead for the connection. From the perspective of minimizing processing overhead and associated communication latency, the fewer hops in a connection, the better. From this perspective, star networks are attractive because each connection in the network has only one hop or two hops maximum.
One of the disadvantages of star networks is that the central node is involved in all network traffic. In particular, for each star network connection, the central node is either a terminal node (i.e., the start node or the end node for the connection) or an intermediate node (i.e., a pass-through node for a connection between two non-central nodes). As a result, as the number of connections in a star network increases, traffic bottlenecks are more and more likely to occur at the central node.
Another disadvantage of star networks is that the central node corresponds to a potential single point of failure for the entire network. If the central node fails, then all communication in the network will cease.
A star topology is useful for applications in which most or all traffic is to and/or from a single node. In such applications, that node is configured to be the central node in the star network. On the other hand, a star topology is not generally suitable for the backbone of a service provider network, because it creates both a bottleneck and a single point of failure.
To avoid the disadvantages of star networks associated with traffic bottlenecks and single points of failure, networks, such as service provider networks, may be configured with non-star topologies such as a ring topology. In a ring network, every node in the network is directly connected to exactly two other nodes. Ring networks are less susceptible to traffic bottlenecks than star networks because there is no single node in a ring network that is involved in all possible network traffic. Ring networks also avoid the single point of failure problems associated with star networks. In particular, if any particular node or physical link fails in a ring network, then, in accordance with a typical ring-based protection scheme, any traffic associated with that failed node or link may simply be routed around the ring in the other direction (assuming that sufficient bandwidth is available).
On the other hand, one of the disadvantages of ring networks as compared to star networks is that the minimum distance between two nodes on diametrically opposing sides of an n-node ring network is n/2 hops. As such, as the number of nodes in a ring network increases, the average amount of processing overhead per network connection also increases.
It should be noted that the previous discussion focuses on ideal topologies and ideal advantages and disadvantages thereof. In practice, not all ring nodes are equivalent, nor are all of the spoke nodes of a star equivalent. For example, in some situations, one or more ring nodes in a ring or hub or spoke nodes in a star may typically be connected to one or more network feeders, potentially in a sparing arrangement. A feeder-connected node is typically equipped with additional redundant hardware for improved robustness to failure.